Hypernova (pl. hypernovae) refers to an exceptionally large star that collapses at the end of its lifespan—for example, a collapsar, or a large supernova. Up until the 1990s, it had a more specific meaning to refer to an explosion with an energy of over 100 supernovae (1046 joules). Such explosions were proposed to explain the origin of exceptionally bright gamma ray bursts. An extensive sky search found several apparent hypernova remnants, but the frequency was too low to support the hypothesis. Today the term is used to describe the supernovae of the most massive stars, the hypergiants, which have masses from 100 to 150 times that of the Sun. Hypernovae could theoretically pose serious threats to Earth in terms of radiation output, but no stars capable of creating hypernovae have been located near Earth. The word collapsar, short for collapsed star, is an early word for the end product of stellar gravitational collapse, a stellar-mass black hole. The word, used in this sense, is obsolete; but the term "collapsar" now sometimes refers to a specific model for the collapse of a fast-rotating star, as discussed below.
The core of the hypernova collapses directly into a black hole, and two extremely energetic jets of plasma are emitted from its rotational poles at nearly the speed of light. These jets emit intense gamma rays and are a candidate explanation for gamma ray bursts. In recent years a great deal of observational data on gamma ray bursts significantly increased our understanding of these events and made clear that the collapse model produces explosions that differ only in detail from more or less ordinary supernovae. Nevertheless, they continue sometimes to be referred to in the literature as hypernovae. The word hypernova itself was coined by S.E. Woosley.
Since stars sufficiently large to collapse directly into a black hole are quite rare, hypernovae would likewise be rare, if they indeed occur. It has been estimated that a hypernova would occur in our galaxy every 200 million years.
Collapsar is currently used as the name of a hypothetical model where a fast-rotating Wolf-Rayet star with a massive (greater than 30 solar masses) core collapses to form a large, rotating black hole, drawing in the surrounding envelope of stellar matter at relativistic speeds with a Lorentz factor of around 150. These speeds would make collapsars the fastest known celestial objects. They may be considered to be "failed" type Ib supernovae.
It is believed that collapsars are the cause of long (> 2 seconds) gamma-ray bursts, since powerful energy jets would be created along the rotation axis of the black hole, creating a burst of high-energy radiation to an observer whose line of sight is along the jet.
A possible example of a collapsar is the unusual supernova Sn1998bw, which was associated with the gamma-ray burst GRB980425. This was classified as a type Ic supernova due to its distinctive spectral properties in the radio spectrum, indicating the presence of relativistic matter.
Another type of hypernova is a pair-instability supernova, of which SN 2006gy was possibly the first observed example. This supernova event was observed in a galaxy about 240 million light years (72 million parsecs) from Earth. In a pair-instability supernova, the pair production effect causes a sudden pressure drop in the star's core, leading to a rapid partial collapse, which causes a sharp rise in temperature and pressure leading to an explosive thermonuclear burning and complete explosion of the star.
Collapsars in science fiction
- Larry Niven used a massive "collapsar" to create a time travel effect for the short story "Singularities Make Me Nervous."
- Jerry Pournelle used a "collapsar" as a means of causing a ship to drop out of hyperspace unintentionally due to the gravitational effect in the story "He Fell Into a Dark Hole."
- Joe Haldeman used "collapsars" as means of interstellar travel in his novel The Forever War.
References and further reading
- A. I. MacFadyen and S. E. Woosley "Collapsars: Gamma-Ray Bursts and Explosions in 'Failed Supernovae'" Astrophysical Journal, Vol 524, Pages 262–289, October 1999.
- Stanford E. Woosley "Gamma-ray bursts from stellar mass accretion disks around black holes" Astrophysical Journal, Vol 405, Pages 273–277, March 1993.
- Tsvi Piran "The Physics of Gamma-Ray Bursts" Reviews of Modern Physics, Vol 76, October 2004
- Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked European Southern Observatory (ESO)
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